Circuit board and method of manufacturing the same

ABSTRACT

A circuit board and a method of manufacturing the same are provided. The circuit board includes: a multilayer board in which a plurality of conductive layers with desired patterns formed therein, and a plurality of insulating layers are stacked; a plurality of through holes penetrating the multilayer board; cylindrical recesses each formed around a through hole corresponding thereto, having a diameter larger than that of the through hole, having a depth from an outermost surface of the insulating layer to a surface of the conductive layer for electrical connection, and partially exposing the surface of the predetermined conductive layer; and a plurality of conductive terminals fitted into the through holes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2006-235609 filed on Aug. 31,2006, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit board and a method ofmanufacturing the same, and more specifically, to a circuit board, whichallows a large electric current to flow through a circuit patterntherein, and a method of manufacturing the same.

2. Description of the Related Art

In general, use of electronic circuits has been rapidly expanded to suchfields as industry machines and automobile parts. In particular, acomposite circuit board has been widely used in an inverter circuit, acontrol circuit of a servo motor, a power supply unit, or the like. Inthe composite circuit board, a thin circuit pattern for a small current(for a signal) and a thick circuit pattern for a large current areformed in one insulating board. Various circuit boards having suchcomposite circuits have been known.

FIG. 8 is a cross sectional view showing a large current circuit boardaccording to a conventional technology. As shown in FIG. 8, a largecurrent circuit board 60 includes an insulating board 61 made of glassepoxy material or the like; conductors 62,63 formed with copper usedtherein; a control circuit conductor 64 made of copper by etching bothsides of the insulating board 61 or the like; and a through hole part 65which penetrates the insulating board 61 and the conductors 62,63.Interfaces 67 between the through hole part 65 and the conductors 62,63are copper-plated, respectively, thus forming a predetermined currentconductor path through the conductors 62,63,64. Voltages having oppositepolarities are applied to the conductors 62,63. In the large currentcircuit board 60, an ordinary conductor has a thickness of about 35 μm.

FIG. 9 and FIG. 10 are a cross sectional view and a perspective view,respectively, each showing another large current circuit board accordingto a conventional technology. As shown in FIG. 9 and FIG. 10, a largecurrent circuit board 70 is formed such that different burred terminals75 are electrically connected on an upper surface of the board 70 via athick conductor 74 (see, for example, Japanese Laid-Open PatentApplication, Publication No. H10-56244, claim 1, FIG. 1).

Namely, the large current circuit board has a signal conductor 72 andpatterned conductors 73 formed by attaching copper foil to a surface 71a of the insulating board 71 made of glass epoxy material or the like,and etching the copper foil into a predetermined pattern. The thickconductor 74 is formed by cutting a copper sheet into a predeterminedpattern, and is soldered to the pattered conductor 73, which is etchedin a shape corresponding to the large current thick conductor 74.

In the large current circuit board 70, the burred terminal 75 burred ina cylindrical shape is inserted into the through hole part 76, whichpenetrates the insulating board 71. Then an end 75 a of the burredterminal 75 is soldered to a patterned conductor 77, which is formed onan underside 71 b of the insulating board 71. As shown in FIG. 10, whena plurality of different burred terminals 75 are electrically connected,a plurality of thick conductors 74 each in a rod shape with an insulatedsurface thereof are disposed one above another. This means that thelarge current circuit board 70 has a larger volume in a height directionthereof, as a circuit thereof becomes more complicated. In the largecurrent circuit board 70, an ordinary conductor has a thickness of about210 to 500 μm.

The large current circuit board 60 shown in FIG. 8 is not suitable,however, for flowing a large current therethrough, because an area ofthe interfaces 67 between the through hole part 65 and the conductors62,63 are substantially small. Nonetheless, reduction of a combinedinductance (a surge reduction) can be expected, since the conductors62,63 in the insulating board 61 are provided close to each other. Toflow a large current through the large current circuit board 60 shown inFIG. 8, it is necessary to provide a large number of the through holeparts 65, to thereby increase a total electrical contact area of theinterfaces 67. However, a more complicated manufacturing process isrequired to provide the large number of the through hole parts 65.

As compared to the large current circuit board 60, the large currentcircuit board 70 shown in FIG. 9 and FIG. 10 is suitable for flowing alarge current therethrough. However, the large current circuit board 70requires a longer time in forming and placing the thick conductors 74therein, because the thick conductors 74 have to be placed between aplurality of the through hole parts 76 for each current path. Further,there may be a region in the large current circuit board 70, in which aplurality of the thick conductors 74 are multilayered in the heightdirection. As a result, an area for placing other components inevitablybecomes smaller, and a thickness of the board becomes larger. Moreover,reduction of a combined inductance (a surge reduction) cannot be so muchexpected in the large current circuit board 70 as in the large currentcircuit board 60 shown in FIG. 8, because a distance between theopposing patterned conductors 73,77 is larger in the large currentcircuit board 70.

The present invention has been made in light of the above-mentionedproblems. The present invention provides a circuit board capable offlowing a large current therethrough with a relatively simpleconfiguration and without increasing a size thereof in the thicknessdirection. The present invention also provides a method of manufacturingthe circuit board.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a circuit boardincludes: a multilayer board, in which a plurality of insulating layersand a plurality of conductive layers are alternately stacked; and aplurality of conductive terminals, which are fitted into a plurality ofthrough holes extending in a thickness direction of the multilayer boardat predetermined positions thereof. The multilayer board includescylindrical recesses each having a diameter larger than that of thethrough hole, having a depth from an outermost insulating layer in themultilayer board to a surface of a predetermined conductive layer in themultilayer board, and partially exposing the surface of thepredetermined conductive layer. The conductive layer and the conductiveterminal are electrically connected at an interface between theconductive layer and the conductive terminal in the through hole, and atan electrical interface between the conductive terminal and thepartially exposed surface of the conductive layer in the predeterminedconductive terminal.

In the aspect of the present invention, in the circuit board having theabove-mentioned configuration, the conductive layer and the conductiveterminal are electrically connected at the interface in the through holeand at the electrical interface on the partially exposed surface of theconductive layer, which is exposed by forming the cylindrical recess. Inthis way, the circuit board can secure a current path having a largeelectrical contact area between the conductive layer and the conductiveterminal, thus enabling the circuit board to flow a large currenttherethrough. Further, because the circuit board has the largeelectrical contact area, when the circuit board flows a large currenttherethrough, it is possible to prevent that the current is concentratedon the through hole, and a temperature at the through hole goes up.

It is to be noted that the cylindrical recess may be formed concentricwith the through hole, or may have a center thereof offset from that ofthe through hole.

According to another aspect of the present invention, in a method ofmanufacturing a circuit board, the circuit board includes: a multilayerboard, in which a plurality of insulating layers and a plurality ofconductive layers are alternately stacked; and a plurality of conductiveterminals, which are fitted into a plurality of through holes extendingin a thickness direction of the multilayer board at predeterminedpositions thereof. The method of manufacturing the circuit boardincludes: a first step of forming the through holes each penetrating themultilayer board in a thickness direction thereof; a second step offorming cylindrical recesses each having a diameter larger than that ofthe through hole, provided by drilling from an outermost insulatinglayer in the multilayer board to a surface of a predetermined conductivelayer in the multilayer board, and partially exposing the surface of thepredetermined conductive layer; a third step of fitting the conductiveterminal into the through hole corresponding thereto; and a fourth stepof connecting via conductive connecting material at least either at aninterface between the conductive layer and the conductive terminal inthe through hole, or at an electrical interface between the conductiveterminal and the partially exposed surface of the conductive layer inthe cylindrical recess.

In another aspect of the present invention, in the method ofmanufacturing the circuit board including the above-mentioned steps, thecylindrical recess is formed before the conductive terminal is fittedinto the through hole, and the conductive terminal is electricallyconnected to the partially exposed surface of the conductive layer,which is exposed by forming the cylindrical recess. The circuit board isheated to a predetermined temperature with the conductive connectingmaterial applied therein, such as cream solder. Then the conductiveterminal fitted into the through hole is electrically connected to theconductive layer at the interface in the through hole and at theelectrically interface in the cylindrical recess. As a result, thecircuit board can secure a current path having a large electricalcontact area between the conductive layer and the conductive terminal.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention, whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically showing a configurationof a circuit board according to a first embodiment of the presentinvention.

FIG. 2A to FIG. 2D are sequential cross sectional views eachschematically showing a key step in a method of manufacturing thecircuit board according to the first embodiment.

FIG. 3A to FIG. 3C are plan views each showing the circuit board of thepresent invention, before a multilayer board is stacked.

FIG. 4A is a plan view showing the circuit board of the presentinvention, to which electronic components are connected. FIG. 4B is across sectional view of FIG. 4A. FIG. 4C is a schematic view forexplaining inductance in the circuit board of FIG. 4A.

FIG. 5A to FIG. 5D are sequential cross sectional views each showing akey step in the method of manufacturing a circuit board according to asecond embodiment.

FIG. 6A to FIG. 6C are sequential cross sectional views each showing akey step of the method of manufacturing a circuit board according to athird embodiment.

FIG. 7A is an exploded perspective view schematically showing apartially broken circuit board according to a fourth embodiment. FIG. 7Bis a perspective view schematically showing the partially broken circuitboard according to the fourth embodiment.

FIG. 8 is a cross sectional view showing a large current circuit boardaccording to related art.

FIG. 9 is a cross sectional view showing another circuit board accordingto related art.

FIG. 10 is a perspective view schematically showing the large currentcircuit board of FIG. 9.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

With reference to attached drawings, exemplary embodiments of thepresent invention are described next in detail.

First Embodiment

<General Configuration of Circuit Board>

As shown in FIG. 1, the circuit board 1 includes a multilayer board 11,conductive terminals 15A,15B, and through hole pins 14 c,14 d.

The multilayer board 11 is formed by alternately stacking insulatinglayers 13 a,13 b,13 c and conductive layers 12 a,12 b. The insulatinglayers 13 a,13 b,13 c are thin layers made of insulating material suchas glass epoxy resin. The conductive layers 12 a,12 b are thin layersmade of copper, for example, and have predetermined circuit patternstherein.

The insulating layers 13 a,13 b,13 c and the conductive layers 12 a,12 bhave each suitable thickness according to material thereof, a magnitudeof a current which is to flow through the conductive layers 12 a,12 b,and a size of the circuit board 1.

The multilayer board 11 has cylindrical through holes 14 a,14 b, whichpenetrate the multilayer board 11 at predetermined positions thereof;and counterbored holes (cylindrical recesses) 17 a,17 b to be describedlater, which penetrate the insulating layer 13 a or the insulatinglayers 13 a,13 b so as to reach the conductive layers 12 a,12 b.Cylindrical through hole pins 14 c,14 d made of copper are fitted intothe through holes 14 a,14 b, respectively. Conductive terminals 15A,15Bmade of copper are provided on respective inner circumferences of thethrough hole pins 14 c,14 d.

An insulating part g1 is provided around a portion of the counterboredhole 17 b, which is in contact with the conductive layer 12 b, so as toensure insulation between the conductive terminal 15B and the conductivelayer 12 a. An insulating part g2 is provided around a portion of thethrough hole 14 a, which is in contact with the conductive layer 12 a,so as to ensure insulation between the conductive terminal 15A and theconductive layer 12 b. The insulating part g1 has a larger cross sectionthan the counterbored hole 17 b (see FIG. 3B). The insulating part g2has a larger cross section than the counterbored hole 14 a (see FIG.3B). It is to be noted that, in FIG. 3A to FIG. 3C, markings M1 indicateouter circumferences of the through holes 14 a,14 b, and markings M2indicate outer circumferences of the counterbored holes 17 a,17 b.

<Through Hole>

As shown in FIG. 1, the through holes 14 a,14 b are cylindrical holespenetrating the multilayer board 11 in a thickness direction thereof.When the counterbored hole 17 a,17 b to be described later are formed,the through hole 14 a penetrates the conductive layers 12 a,12 b and theinsulating layers 13 b,13 c, and the through hole 14 b penetrates theconductive layer 12 b and the insulating layer 13 c.

<Counterbored Hole>

As shown in FIG. 1, the counterbored holes 17 a,17 b are cylindricalholes, are concentric with the through holes 14 a,14 b, and are largerthan those in diameter, respectively. The counterbored hole 17 a isformed to penetrate the insulating layer 13 a, to thereby expose apartial upper surface (a partially exposed surface) 16 a of theconductive layer 12 a. The counterbored hole 17 b is formed to penetratethe insulating layer 13 a, conductive layer 12 a, and insulating layer13 b, to thereby expose a partial upper surface (a partially exposedsurface) 16 b of the conductive layer 12 b.

<Through Hole Pin>

The through hole pins 14 c,14 d are cylindrical members and are made ofconductive material such as copper. The through hole pins 14 c,14 d havediameters to come in contact with the conductive layers 12 a,12 b in thethrough holes 14 a,14 b, respectively. The through hole pins 14 c,14 dhave lengths corresponding to depths of the through holes 14 a,14 b,respectively.

<Conductive Terminal>

The conductive terminals 15A,15B are made of conductive material such ascopper. The conductive terminals 15A,15B include flange parts 18 a,18 b,which are larger than the through holes 14 a,14 b in diameter; and bodyparts 15 a,15 b, which are smaller than the flange parts 18 a,18 b indiameter, respectively. The conductive terminals 15A,15B have thereincylindrical holes (communication holes) 15 c,15 d penetrating upper endsthrough lower ends of the conductive terminals 15A,1513, respectively.The body parts 15 a,15 b have diameters to fit into the through holepins 14 c,14 d, respectively. The flange part 18 a has a diametersmaller than that of the counterbored hole 17 a, and is electricallyconnected to the partial upper surface 16 a of the conductive layer 12 aat an electrical interface 19 b. The flange part 18 b has a diametersmaller than that of the counterbored hole 17 b, and is electricallyconnected to the partial upper surface 16 b of the conductive layer 12 bat another electrical interface 19 b. The flange parts 18 a,18 b and thebody parts 15 a,15 b have lengths corresponding to depths of thecounterbored holes 17 a,18 b and the through holes 14 a,14 b,respectively.

<Conductive Connecting Material>

Conductive connecting material 19 is provided between the body part 15 aand the through hole pin 14 c, between the body part 15 b and thethrough hole pin 14 d, and at the electrical interfaces 19 b. Theconductive connecting material 19 in the present embodiment is solder,which may be provided by known methods such as dip soldering and creamsoldering. The conductive connecting material 19 fills gaps between theconductive terminals 15A,15B, through hole pins 14 c,14 d, andconductive layers 12 a,12 b, while keeping conductivity therebetween.The conductive connecting material 19 also fixes the conductiveterminals 15A,15B into the multilayer board 11.

<Electrical Connection>

In the circuit board 1 having a configuration as described above, theconductive terminal 15A and the conductive layer 12 a are electricallyconnected, and the conductive terminal 15B and the conductive layer 12 bare electrically connected. More specifically, the conductive layer 12 ais electrically connected not only to the through hole pin 14 c at aninterface 19 a, but also to the conductive terminal 15A at theelectrical interface 19 b. This makes a surface area of a current pathlarger, because the flange part 18 a of the conductive terminal 15Acontacts the partial upper surface 16 a of the conductive layer 12 a viathe conductive connecting material 19. Further, the conductive layer 12b is electrically connected not only to the through hole pin 14 d atanother interface 19 a, but also to the conductive terminal 15B atanother electrical interface 19 b. This makes the surface area of thecurrent path larger, because the flange part 18 b of the conductiveterminal 15B contacts the partial upper surface 16 b of the conductivelayer 12 b via the conductive connecting material 19.

Herein, in the circuit board 1, voltages having opposite polarities areapplied to the conductive layers 12 a,12 b. In a predetermined region inthe circuit board 1, currents in opposite directions flow through theconductive layers 12 a,12 b, which are provided close to each other.Hence, magnetic fluxes generated by the currents flowing through theconductive layers 12 a,12 b are cancelled from each other, thus reducinga combined inductance (or a surge) (see FIG. 4).

<Variations of Configuration>

In the circuit board 1, materials and shapes of the insulating layers 13a,13 b,13 c, conductive layers 12 a,12 b, conductive terminals 15A,15B,through hole pins 14 c,14 d are not limited to those shown in FIG. 1.However, any other materials and shapes may be used. In FIG. 1, the twoconductive layers 12 a,12 b are provided. However, three or moreconductive layers may be provided. In FIG. 1, the two conductiveterminals 15A,15B are provided. However, three or more conductiveterminals may be provided according to the number of the conductivelayers, or of electronic components (see FIG. 4) to be mountedafterwards for making the circuit board 1 a finished product.

In the circuit board 1, gaps are created around outer circumferences ofthe flange parts 18 a,18 b, because the counterbored holes 17 a,17 bhave larger diameters than those of the flange parts 18 a,18 b of theconductive terminals 15A,15B, respectively. The gaps facilitatesoldering (insertion of a soldering iron into the counterbored holes 17a,17 b). However, another configuration is possible, in which thecounterbored holes 17 a,17 b have substantially same diameters as thoseof the conductive terminals 15A,15B, respectively, and the insulatinglayer 13 a or the like comes in contact with the outer circumferences ofthe flange parts 18 a,18 b without gaps.

In the circuit board 1, the upper ends of the conductive terminals15A,15B are positioned higher than an upper surface of the insulatinglayer 13 a. Since the conductive terminals 15A,15B stand higher and thegaps are created around the inner circumferences of the counterboredholes 17 a,17 b, electronic components to be described later (see FIG.4) can be connected onto the conductive terminals 15A,15B more easily.However, the upper ends of the conductive terminals 15A,15B may bepositioned equal to or lower than the upper surface of the insulatinglayer 13 a, according to a size of an electronic component to be mountedor a height limit of a space in which the circuit board 1 is used.

Instead of the cylindrical holes 15 c,15 d, the conductive terminals15A,15B may have grooves for screwing up the electronic components to bedescribed later.

It is to be noted that, in the circuit board 1, a single conductivelayer has a thickness of about 400 μm, for example, and a singleinsulating layer has a thickness of about 500 μm, for example.

<Steps in Method of Manufacturing Circuit Board>

Next is described a method of manufacturing the circuit board 1 mainlywith reference to FIG. 2A to FIG. 2D. FIG. 2A to FIG. 2D are sequentialcross sectional views each schematically showing a key step in themethod of manufacturing the circuit board 1 of FIG. 1. FIG. 3A to FIG.3C are plan views showing each layer of the circuit board 1. FIG. 4A isa plan view showing the circuit board 1, to which the electroniccomponents are connected. FIG. 4B is a cross sectional view of FIG. 4A.FIG. 4C is a schematic view for explaining inductance in the circuitboard 1 of FIG. 4A. It is to be noted that FIG. 1 is a cross sectionalview when cut along the line X1-X1 of, for example, FIG. 3A, FIG. 3B orFIG. 3C.

First, the multilayer board 11 is manufactured. The insulating layers 13a,13 b,13 c are prepared separately, as shown in FIG. 3A, FIG. 3B andFIG. 3C, respectively. The prepared insulating layers 13 a to 13 c areintegrally stacked such that the insulating layer 13 a is in anuppermost position; the insulating layer 13 b is in an intermediateposition with the conductive layer 12 a provided thereon; and theinsulating layer 13 c is in a lowermost position with the conductivelayer 12 b provided thereon. Thus the multilayer board 11 ismanufactured as shown in FIG. 2A. The multilayer board 11 is pressed tohave a multilayered structure, in which the layers 13 a,13 b,13 c and 12a,12 b are closely stuck to each other. Round holes 82 formed atrespective four corners of the insulating layers 13 a,13 b,13 c arescrew holes used for screwing up the entire circuit board 1. Small holes84 penetrating the respective layers at predetermined positions areterminal holes for soldering connectors and electrical wires.

In the multilayer board 11, the markings M1 for indicating outercircumferences of through holes and the markings M2 for indicating thoseof counterbored holes are provided on the uppermost insulating layer 13a, as shown in FIG. 3A. This makes it possible for an operator tovisually check where to drill the holes. Either or both of the markingsM1,M2 are also provided on the conductive layer 12 a and insulatinglayer 13 b, as shown in FIG. 3B and FIG. 3C. It is assumed that aposition, a depth, and the like to be drilled by drills d1,d2 (notshown) used herein are numerically controlled.

As shown in FIG. 2A, a first step of manufacturing the circuit board 1is a step of forming the through holes 14 a,14 b. The through holes 14a,14 b are formed to mechanically penetrate the multilayer board 11 in athickness direction thereof at respective predetermined positions.Herein, numerically-controlled small-diametered drills d1,d1 create thethrough holes 14 a,14 b simultaneously or separately.

As shown in FIG. 2B, a second step is a step of forming the counterboredholes 17 a,17 b. In forming the counterbored hole 17 a, anumerically-controlled large-diametered drill d2 is used to drill ahole, such that the hole reaches the partial upper surface 16 a of theconductive layer 12 a and is concentric with the through hole 14 a. Informing the counterbored hole 17 b, a numerically-controlledlarge-diametered drill d2 is also used to drill a hole, such that thehole reaches the partial upper surface 16 b of the conductive layer 12 band is concentric with the through hole 14 b. The drill d2 used hereinhas a flat tip thereof. It is to be noted that, in the present art, adiameter of 0.5 mm or less may be regarded as a “small diameter”, and adiameter of more than 0.5 mm may be regarded as a “large diameter”.However, the “small diameter” and the “large diameter” are hereinreferred to just for relatively indicating the diameter of the drill d1(for example, about 6.0 mm) and that of the drill d2 (for example, about12.0 mm), respectively.

As shown also in FIG. 2B, an insertion step is performed between thesecond and third steps. In the insertion step, the through hole pins 14c,14 d are inserted into the through holes 14 a,14 b, respectively. Theinsertion is carried out automatically by an automated operating machineor manually by an operator, both not shown. The through hole pins 14c,14 d are provided to facilitate and ensure insertion of the conductiveterminals 15A,15B, respectively. However, the through hole pins 14 c,14d are not indispensably required and may be omitted. Namely, theconductive terminals 15A,15B may be inserted directly into the throughholes 14 a,14 b, respectively. Instead of using the through hole pins 14c,14 d, inner surfaces of the through holes 14 a,14 b may be metalplated. In this case, for example, the through holes 14 a,14 b may befirst created in the multilayer board 11 using the drill d1. Entiresurfaces of the through holes 14 a,14 b are then subjected tononelectrolytic copper plating. After that, the counterbored holes 17a,17 b are created using the drill d2.

As shown in FIG. 2C, a third step is a step of fitting the conductiveterminals 15A,15B into the multilayer board 11. More specifically, theconductive terminals 15A,15B are set such that the body parts 15 a,15 bthereof are fitted into the through hole pins 14 c,14 d, respectively,and the flange parts 18 a,18 b thereof come in contact with the partialupper surfaces 16 a,16 b, respectively. Herein, the partial uppersurfaces 16 a,16 b are in exposed states by the counterbored holes 17a,17 b, respectively. The conductive terminals 15A,15B are setautomatically by an automated operating machine or manually by anoperator, both not shown. Alignment of the conductive terminals 15A,15Bis easily performed by inserting the conductive terminals 15A,15B allthe way into the through hole pins 14 c,14 d, because the conductiveterminals 15A,15B have the flange parts 18 a,18 b, and the body parts 15a,15 b have lengths corresponding to depths of the through hole pins 14c,14 d, respectively.

As shown in FIG. 2D, a fourth step is a step of applying the conductiveconnecting material 19. Herein, cream solder is used as the conductiveconnecting material 19, and reflow soldering with cream solder is usedas a soldering method. The conductive connecting material 19 is appliedto circumferential edges of the flange parts 18 a,18 b, which are shownin dotted lines in FIG. 2D. The conductive connecting material 19 flowsinto respective gaps between the conductive terminals 15A,15B,counterbored holes 17 a,17 b, through holes 14 a,14 b, and through holepins 14 c,14 d. It would be preferable that an amount of the conductiveconnecting material 19 to be applied is sufficient to ensure largeelectrical contact areas between side surfaces of the flange parts 18a,18 b and the partial upper surfaces 16 a,16 b, respectively, evenafter the conductive connecting material 19 is heated and melted to flowinto the gaps. Heating and melting the conductive connecting material 19is to be described later.

As shown in FIG. 2D, a fifth step is a step of heating the circuit board1 with the conductive connecting material 19 applied thereon, to apredetermined temperature using a heating furnace not shown. Theconductive connecting material 19 is heated and melted to flow into gapscreated between the flange parts 18 a,18 b and counterbored holes 17a,17 b, between the body parts 15 a,15 b and through hole pins 14 c,14d, or the like. The conductive connecting material 19 in the gaps formsthe interfaces 19 a and the electrical interfaces 19 b, thus ensuringelectrical connections between the conductive terminals 15A,15B andconductive layers 12 a,12 b, respectively. The conductive connectingmaterial 19 also resultantly fixes the above-mentioned members.

The soldering used herein is not limited to the reflow soldering asdescribed above, and may be any known flow soldering. If the flowsoldering is used, the cylindrical holes 15 c,15 d are preferablycovered with a polyimide film-based heat-resistant masking tape or thelike, before the soldering. This ensures protection of insides of thecylindrical holes 15 c,15 d shown in FIG. 1.

When the conductive terminals 15A,15B are set, the flange parts 18 a,18b thereof come in contact with the partial upper surfaces 16 a,16 b (seeFIG. 2C), which are exposed by the counterbored hole 17 a,17 b,respectively. This results in large electrical contact areas between theconductive terminals 15A,15B and the conductive layers 12 a,12 b,respectively, thus increasing a surface area of a current path.

Next are described electronic components Db mounted in the circuit board1 with reference to FIG. 4A and FIG. 4B. It is assumed herein that apower supply circuit (not shown) is connected to the conductiveterminals 15A,15B via screws na,na, respectively, and electroniccomponents Db are connected to another conductive terminals 15A,15B viascrews nb,nb, respectively. The electronic components Db may be aswitching element (for example, IGBT, MOSFET, or the like), a capacitor,a snubber circuit, or the like.

FIG. 4B is a cross sectional view showing the circuit board 1, when FIG.4A is cut along the line X2-X2. FIG. 4C is a schematic view showing anelectrical circuit when an electric current flows through the circuitboard 1 of FIG. 4. In FIG. 4B, the conductive terminals 15A,15B on aleft side of the figure have taps (internal screws) on upper portions ofthe cylindrical holes 15 a,15 b thereof, respectively, and are directlyscrewed with the screws na,na, which are connected to the power supplycircuit. The conductive terminals 15A,15B on a right side of the figuredoes not have taps on upper portions of the cylindrical holes 15 c,15 d,but have taps inside screws nb,nb, and are connected to the electroniccomponents Db.

In a configuration shown in FIG. 4A and FIG. 4B, a power supply isturned on, and electric currents flow through current paths Sa,Sb asindicated by arrows in FIG. 4B. When a voltage of, for example, 600V isapplied, electric currents of 200 A to 300 A flow therethrough. In thiscase, as shown in FIG. 4B, when voltages having opposite polarities areapplied to the conductive layers 12 a,12 b, which are positioned aboveand below across the insulating layer 13 b, electric currents inopposite directions flow therethrough. The opposite electric currentscancel respective magnetic fields generated by themselves. Next isdescribed further the above phenomenon with reference to FIG. 4C. In thecircuit board 1 according to the present invention as shown in FIG. 4C,as well as in the circuit boards in the aforementioned prior art withreference to FIG. 8 to FIG. 10, inductance components L1,L2 are presentbetween the electronic components Db, and a positive power supply and anegative power supply, respectively. A combined inductance L of theentire circuit board 1 is represented as L=(L1+L2±2M). Herein, M is amutual inductance. However, in the circuit board 1 according to thepresent invention, the conductive layers 12 a,12 b are provided close toeach other, and directions of the current paths Sa,Sb are opposite toeach other. As a result, the magnetic fields generated by electriccurrents flowing through the conductive layers 12 a,12 b are cancelled,thus reducing a combined inductance (a surge reduction) to asubstantially negligible extent.

As described above, the circuit board 1 according to the presentinvention ensures electrical connections not only at the interfaces 19 abut also at the electrical interfaces 19 b between the conductiveterminal 15A and the conductive Layers 12 a,12 b and between theconductive terminal 15B and the conductive layers 12 a,12 b. A largeelectrical contact: area therebetween in the present invention preventsa possible trouble caused by a flow of a large electric current owing toa local heat generation or the like. Thus the circuit board 1 issuitable for flowing a large electric current therethrough. Further, thepresent invention uses the counterbored holes and the conductiveconnecting material 19. This eliminates a need for an enlargement of asize of the circuit board 1 or processing of a number of through holes,which is otherwise required to flow a large electric current. Moreover,the present invention can be realized with a relatively simple processand configuration. The present invention also achieves an inductancereduction by providing the conductive layers 12 a,12 b close to eachother in the multilayer board 11. It is to be noted that the electricalcontact area at the interfaces 19 a or the electrical interfaces 19 bmay be determined according to a size of the circuit board 1, amagnitude of an electric current to flow therethrough, or the like.

Second Embodiment

Next is described a second embodiment with reference to FIG. 5A to FIG.5D. FIG. 5A to FIG. 5D are sequential cross sectional views showing afirst step to a fifth step as key steps in the method of manufacturing acircuit board. A configuration of the second embodiment is the same asthat of the first embodiment shown in FIG. 1, except that the number oflayers stacked in the circuit board is different, and that a pair ofcounterbored holes are formed on both an upper surface and a lowersurface of the circuit board. Descriptions of a configuration and aprocess which have been already mentioned in the first embodiment willbe omitted herefrom.

<General Configuration of Circuit Board>

As shown in FIG. 5C and FIG. 5D, a circuit board 2 according to thesecond embodiment includes: a multilayer board 21, through hole pins 24c,24 d, and conductive terminals 32A,32B, latter two of which are placedin the multilayer board 21.

The multilayer board 21 is configured such that insulating layers 23a,23 b,23 c,23 d,23 e and conductive layers 22 a,22 b, 22 c,22 d arealternately stacked.

The multilayer board 21 has cylindrical through holes 24 a,24 b, whichpenetrate the multilayer board 21 at predetermined positions thereof;first counterbored holes 27 a,27 b, formed to reach from the insulatinglayer 23 a to top surfaces of the conductive layers 22 c,22 d,respectively; and second counterbored holes 29 a,29 b formed to reachfrom the insulating layer 23 e to lower surfaces of the conductivelayers 22 c,22 d, respectively.

The multilayer board 21 has an insulating part g4 for ensuringinsulation between a conductive terminal 32A and the conductive layer 22b; an insulating part g6 between the conductive terminal 32A and theconductive layer 22 d; and an insulating part g3 between a conductiveterminal 32B and the conductive layer 22 a; and an insulating part g5between the conductive terminal 32B and the conductive layer 22 c.

<Electrical Connection between Second Counterbored Hole and ConductiveTerminal>

As shown in FIG. 5D, body parts 32 a,32 b of the conductive terminals32A,32B partly protrude from lower ends of the through holes 24 a,24 b,respectively, when the conductive terminals 32A,32B are set in thethrough holes 24 a,24 b via the through hole pins 24 c,24 d,respectively. This makes an electrical contact area between theconductive terminals 32A,32B and partial lower surfaces 26 c,26 dlarger, because the conductive terminals 32A,32B contact the partiallower surfaces 26 c,26 d, respectively, via conductive connectingmaterial 34 applied to respective protruding portions of the body parts32 a,32 b.

In other words, the conductive terminals 32A,32B have large electricalcontact areas between flange parts 31 a,31 b thereof and partial uppersurfaces 26 a,26 b, and between the protruding portions of the bodyparts 32 a,32 b and the partial lower surfaces 26 c,26 d, respectively.Thus the circuit board 2 is suitable for flowing a large electriccurrent therethrough.

Lower ends of the conductive terminals 32A,32B protrude from a lowersurface of the insulating layer 23 e. Further, gaps are created onrespective inner circumferences of the second counterbored holes 29 a,29b. Thus, the electronic components Db (see FIG. 4) can be easilyconnected to the conductive terminals 32A,32B. Herein, the lower ends ofthe conductive terminals 32A,32B may be flush with or lower than thelower surface of the insulating layer 23 e, according to a size of eachof the electronic component Db, a height limit in a space where thecircuit board 2 is used, or the like.

<Steps of Method of Manufacturing Circuit Board>

Next are described steps of a method of manufacturing the circuit board2. Description of the steps which have already mentioned in the firstembodiment shown in FIG. 2 will be simplified herein.

First, the multilayer board 21 is manufactured. Next, as a first step ofthe method, the through holes 24 a,24 b (see FIG. 5B) are created asshown in FIG. 5A. The through holes 24 a,24 b are formed by penetratingthe multilayer board 21 in a thickness direction thereof withnumerically-controlled small-diametered drills d3,d3.

As shown in FIG. 5B, a second step is a step of forming the firstcounterbored holes 27 a,27 b and the second counterbored holes 29 a,29 bin the multilayer board 21. In forming the first counterbored holes 27a,27 b, numerically-controlled large-diametered drills d4,d4 are used todrill holes, such that the holes reach a partial upper surface 26 a ofthe conductive layer 22 a and a partial upper surface 26 b of theconductive layer 22 b, and are concentric with the through holes 24 a,24b, respectively. In forming the counterbored holes 29 a,29 b, thenumerically-controlled large-diametered drills d4,d4 are also used todrill holes, such that the holes reach the partial lower surface 26 c ofthe conductive layer 22 c and the partial lower surface 29 d of theconductive layer 22 d, and are concentric with the through holes 24 a,24b, respectively. The drills d4,d4 used herein each has a flat tipthereof.

As shown in FIG. 5C, a third step is a step of fitting the conductiveterminals 32A,32B into the multilayer board 21. Prior to the third step,however, an insertion step is herein performed, in which the throughhole pins 24 c,24 d are inserted into the through holes 24 a,24 b,respectively. The through hole pins 24 c,24 d are provided to facilitateand ensure insertion of the conductive terminals 32A,32B, respectively.However, the through hole pins 24 c,24 d are not indispensably requiredand may be omitted. Namely, the conductive terminals 32A,32B may beinserted directly into the through holes 24 a,24 b, respectively.Instead of using the through hole pins 24 c,24 d, inner surfaces of thethrough holes 24 a,24 b may be metal plated.

The conductive terminals 32A,32B have flange parts 31 a,31 b, and thebody parts 32 a,32 b have lengths corresponding to depths of the secondcounterbored holes 29 a,29 b, respectively. Therefore, alignment of theconductive terminals 32A,32B can be easily performed just by insertingthe conductive terminals 32A,32B all the way into the through holes 24a,24 b (in accordance with the through hole pins 24 c,24 d) so thatlower ends of the flange parts 31 a,31 b come in contact with thepartial upper surfaces 26 a,26 b, respectively.

As shown in FIG. 5D, a fourth step is a step of applying the conductiveconnecting material 34 around the flange parts 31 a,31 b of theconductive terminals 32A,32B and the body parts 32 a,32 b protrudingfrom the through holes 24 a,24 b, respectively. A fifth step is a stepof heating the circuit board 2 to a predetermined temperature in areflow furnace or the like not shown. The conductive connecting material34 melted by heating the circuit board 2 is flown into gaps between theconductive terminal 32A, the through hole pin 24C, and the through hole24 a, and between the conductive terminal 32B, the through hole pin 24d, and the through hole 24 b. After the conductive connecting material34 is sufficiently spread into the gaps, the conductive terminals32A,32B are set into the through holes 24 a,24 b, respectively, withlarge electrical contact areas obtained therebetween.

In this step, the conductive connecting material 34 formsconically-shaped electrical interfaces 36 a to 36 d between the partialupper surfaces 26 a,26 b of the conductive layers 22 a,22 b and theflange parts 31 a,31 b, and between the body parts 21 a,32 b protrudingfrom the through holes 24 a,24 b and the partial lower surfaces 26 c,26d of the conductive layers 22 c,22 d, respectively. The electricalinterfaces 36 a to 36 d, and the interfaces 35 a,35 b between theconductive terminals 32A,32B and the through holes 24 a,24 b provide acurrent path having a large electrical contact area, thus allowing alarge electric current to flow through the circuit board 2.

In the second step, the first counterbored holes 27 a,27 b may be formedfirst, and then, the second counterbored holes 29 a,29 b may be formed,or vice versa. Or, the first counterbored holes 27 a,27 b and the secondcounterbored holes 29 a,29 b may be formed simultaneously.

The circuit board 2 manufactured as described above can flow a largeelectric current therethrough with a relatively simple configurationthereof and without increasing a size thereof.

Further, in this embodiment as shown in FIG. 5A to FIG. 5D, voltageshaving opposite polarities are applied to the conductive layers 22 a,22b (namely, directions of electric currents flowing therethrough areopposite to each other). Voltages having opposite polarities are alsoapplied to the conductive layers 22 c,22 d. This results in reduction ofa combined inductance (a surge) in the circuit board 2, as in the firstembodiment.

Third Embodiment

Next are described a circuit board and key steps of a method ofmanufacturing the circuit board according to a third embodiment withreference to FIG. 6A, FIG. 6B and FIG. 6C. FIG. 6A, FIG. 6B and FIG. 6Care sequential cross sectional views each showing a key step of themethod of manufacturing the circuit board according to the thirdembodiment.

A configuration of the third embodiment is the same as that of theembodiments described above, except that no through hole pin is used,and that second flange parts are provided in the third embodiment.Difference between the third and the other embodiments is mainlydescribed herein, and description of same configurations is omittedherefrom.

<General Configuration of Circuit Board>

As shown in FIG. 6A, FIG. 6B and FIG. 6C, a circuit board 3 includes amultilayer board 41; first counterbored holes 47 a,47 b and secondcounterbored holes 49 a,49 b each formed concentric with through holes44 a,44 b, respectively, the through holes 44 a,44 b penetrating themultilayer board 41 at predetermined positions thereof; and conductiveterminals 52A,52B set into the through holes 44 a,44 b via the firstcounterbored holes 47 a,47 b and the second counterbored holes 49 a,49b, respectively. Conductive layers 42 a,42 b,42 c,42 d have insulatingparts g7,g8,g9,g10 around the first counterbored holes 47 b, the throughholes 44 a,44 b, and the second counterbored hole 49 a, respectively.

The conductive terminals 52A,52B include first flange parts 51 a,51 bthereof integrally formed on one ends of body parts 52 a,52 b thereof,respectively. The conductive terminals 52A,52B also include secondflange parts 54 a,54 b, which can be freely attached to and removed fromthe other ends of the body parts 52 a,52 b, respectively.

The second flange parts 54 a,54 b are formed in ring shapes havinglarger diameters than those of the body parts 52 a,52 b, respectively.The second flange parts 54 a,54 b and the body parts 52 a,52 b are madeof the same material (copper, for example). The second flange parts 54a,54 b are formed to have depths corresponding to the secondcounterbored holes 49 a,49 b, respectively. The second flange parts 54a,54 b are also formed to have lengths enough to come in contact withthe conductive layers 42 c,42 d exposed by the second counterbored holes49 a,49 b, respectively, and enough to slightly protrude from anundermost surface of the multilayer board 41, when the second flangeparts 54 a,54 b are set from the other ends of the body parts 52 a,52 b,respectively.

In the configuration described above, an electrical contact area in thecircuit board 3 can be made large, because the first flange parts 51a,51 b come in contact with the partial upper surfaces 46 a,46 b of theconductive layers 42 a,42 b exposed by the first counterbored holes 47a,47 b, and the second flange parts 54 a,54 b come in contact with thepartial lower surfaces 46 c,46 d of the conductive layers 42 c,42 dexposed by the second counterbored holes 49 a,49 b, respectively.

<Steps of Method of Manufacturing Circuit Board>

Next is described the method of manufacturing the circuit board 3.

The same steps as those described above in FIG. 5 are used in the thirdembodiment for forming the multilayer board 41, through holes 44 a,44 b,first counterbored holes 47 a,47 b, and second counterbored holes 49a,49 b.

As shown in FIG. 6A, after the first counterbored holes 47 a,47 b andsecond counterbored holes 49 a,49 b are created, conductive connectingmaterial 55 such as cream molder is applied to portions of theconductive layers 42 a to 42 d, which are exposed by the through holes44 a,44 b, the first counterbored holes 47 a,47 b, and the secondcounterbored holes 49 a,49 b, respectively (a third step).

Then the conductive terminals 52A,52B are placed into the through holes44 a,44 b (a fourth step and a fifth step). The conductive terminals52A,52B are fitted thereinto in a state where the second flange parts 54a,54 b are removed. Then, as shown in FIG. 6B, the second flange parts54 a,54 b are fitted into the body parts 52 a,52 b protruding from thefirst counterbored holes 47 a,47 b of the fitted conductive terminals52A,52B, respectively.

As shown in FIG. 6C, after the conductive terminals 52A,52B are set inthe multilayer board 41, the conductive terminals 52A,52B are solderedthereto, by heating the circuit board 3 to a predetermined temperaturein a reflow furnace or the like (not shown) (a sixth step). If thecircuit board 3 is heated with a weight 59 put thereon, the secondflange parts 54 a,54 b hardly move, and are soldered at appropriatepositions. It would be preferable to select a material of the weight 59such that the material ensures contact between the body parts 52 a,52 band the second flange parts 54 a,54 b, does not undergo a chemicalchange or a deformation even at a high temperature during reflowtreatment, and does not damage the multilayer board 41.

In the circuit board 3 manufactured through the steps described above,the conductive connecting material 55 forms electrical interfaces 58 ato 58 d between the first flange parts 51 a,51 b and the conductiveterminals 52A,52B, and between the second flange parts 54 a,54 b, andthe conductive terminals 52A,52B on upper or lower surfaces of theconductive layers 42 a to 42 d. As a result, in the circuit board 3, theelectrical interfaces 58 a to 58 d can provide an electric current pathhaving a large electrical contact area, in which a large electriccurrent can flow through the conductive layers 42 a to 42 d. In thethird embodiment shown in FIG. 6, voltages having opposite polaritiesare applied to the conductive layers 42 a,42 b (namely, directions ofelectric currents flowing therethrough are opposite to each other).Voltages having opposite polarities are also applied to the conductivelayers 42 c,42 d. This results in reduction of a combined inductance (asurge) in the circuit board 3, as in the first embodiment.

The other ends of the body parts 52 a,52 b of the conductive terminals52A,52B may be formed into external screws (not shown), and the secondflange parts 54 a,54 b may be formed into internal screws. In this case,the second flange parts 54 a,54 b may be screwed into the conductiveterminals 52A,52B fitted into the through holes 44 a,44 b, respectively.This ensures connection between the body parts 52 a,52 b and the secondflange parts 54 a,54 b, respectively. Further, the second flange parts54 a,54 b may be just screwed into the body parts 52 a,52 b withoutusing the weight 59.

In the second and third embodiments described with reference to FIG. 5,FIG. 6, respectively, the multilayer boards 21,41 with the firstcounterbored holes created therein may be turned upside down, and may besubjected to processing of forming a lower-positioned secondcounterbored hole or a lower-positioned conductive interface from above.

In FIG. 6, the through hole pins or metal plating in the through holesare not provided. However, if metal plating is provided in the throughholes 44 a,44 b, an increase of an area jointed by the metal platingenhances joint strength of the circuit board 3. Further, metal platingrealizes a more reliable and solderable soldering, when a subsequentsolder joint is performed through reflow treatment or the like, becausesolder can easily spread into minute gaps between the conductiveterminals 52A,52B and the through holes 44 a,44 b.

The above embodiments have configurations in which the counterboredholes (first and second counterbored holes) 17 a (17 b . . . ) areformed as cylindrical recesses (first and second cylindrical recesses);and an electrical contact area is made larger by electrically connectingthe flange part 18 a (18 b . . . ) of the conductive terminal 15A (15B .. . ) to the partial upper surface 16 a (16 b . . . ) or the partiallower surface 26 c (26 d . . . ) of the conductive layer 12 a (17 b . .. ) exposed by the counterbored hole 17 a (17 b . . . ), via theconductive connecting material 19. However, as shown in FIG. 7, theelectrical contact area may be made larger without using the flange part18 a (18 b . . . ).

Fourth Embodiment

FIG. 7A is an exploded perspective view schematically showing apartially broken circuit board according to a fourth embodiment. FIG. 7Bis a view schematically showing the partially broken circuit boardaccording to the fourth embodiment. A configuration of the fourthembodiment shown in FIG. 7A and FIG. 7B is the same as those of theabove embodiments, except that the conductive terminals used in thefourth embodiment do not have any flange parts. Same configurationswhich have already been described in the above embodiments are omittedherefrom.

<Configuration of Circuit Board>

As shown in FIG. 7A and FIG. 7B, a circuit board A1 includes amultilayer board A22; a first counterbored hole (cylindrical recess) A7formed concentric with a through hole A4, the through hole A4 beingprovided at a predetermined position of the multilayer board A22; athrough hole pin A10 fitted into the through hole A4; and a conductiveterminal A5 electrically connected to the through hole pin A10 viaconductive connecting material A9. The conductive terminal A5 usedherein is cylindrical-shaped, thicker than the through hole pin A10, andmade of a conductive material such as copper. An insulating part g11 isprovided around a portion of the through hole A4, which is positionedadjacent to one of conductive layers A2, so as to ensure insulationbetween the conductive layer A2 and the conductive terminal A5.

The multilayer board A22 is configured such that insulating layersA3,A3,A3 and conductive layers A2,A2 are alternately stacked. Thethrough hole A4 is formed at a predetermined position in the multilayerboard A22. A counterbored hole A7 is also formed concentric with thethrough hole A4 in the multilayer board A22, such that a partial uppersurface A6 of the conductive layer A2 exposes from the insulating layerA3. The circuit board A1 has a through hole pin A10 in the through holeA4. In the through hole pin A10, a conductive terminal A5 is placed. Thecircuit board A1 also has conductive connecting material A9 such assolder or cream solder, such that an electrical contact area is madelarge between the partial upper surface A6 of the conductive layer A2exposed by the counterbored hole A7, and an end of the conductiveterminal A5.

The circuit board A1 is configured to have an interface A11 and anelectrical interface A12, which are electrically connected to thepartial upper surface A6 of the conductive layer A2 via the conductiveconnecting material A9 thanks to a formation of the counterbored holeA7. Thus the circuit board A1 has a large electrical contact area, andcan flow a large electric current therethrough with a simpleconfiguration thereof.

<Method of Manufacturing Circuit Board>

In a method of manufacturing the circuit board A1, a step of forming themultilayer board A22, a step of forming the through hole A4, a step ofdrilling the counterbored hole A7, and a step of setting the throughhole pin A10 are performed as aforementioned. In a step of fitting theconductive terminal A5 into the through hole pin A10, the multilayerboard A22 is placed on a flat surface, or the conductive connectingmaterial A9 such as cream solder or the like is applied to an innersurface of the through hole pin A10 as a temporary joint, so as toprevent the conductive terminal A5 from falling off from the throughhole pin A10.

Alignment of the conductive terminals A5 is easily performed just byinserting the conductive terminal A5 all the way into the through holepin A10, because the conductive terminal A5 has a length correspondingto a depth of the through hole pin A10.

After the conductive terminal A5 is fitted into the through hole pinA10, the conductive connecting material A9 is applied such that theelectrical interface A12 is formed for electrically connecting theconductive terminal A5 and the partial upper surface A6 exposed by thecounterbored hole A7, and that an interface A11 is also formed in thethrough hole A4. Then the circuit board A1 is heated to a predeterminedtemperature, to thereby fit the conductive terminal A5 into the circuitboard 1.

<Variations of Configuration>

In FIG. 7, the conductive terminal A5 is fitted into the through hole A4via the conductive through hole pin A10, which is in contact with aninner surface of the through hole A4 as well as an outer surface of theconductive terminal A5. However, the conductive terminal A5 may bedirectly fitted into the through hole A4 without the through hole pinA10. Further, metal plating may be provided in an inner surface of thethrough hole A4, instead of providing the through hole pin A10.

In the present invention, the numbers of the through holes, conductivelayers, and insulating layers are not limited to those described in theabove embodiments. Further, in the present invention, a circuit boardthrough which an electric current of a desired magnitude flows can beconfigured by selecting a thickness and the number of the conductivelayers accordingly.

In the above embodiments, a counterbored hole is formed concentric witha through hole corresponding thereto. However, the counterbored hole ora cylindrical recess may have a center thereof offset from that of thethrough hole. Further, if the cylindrical recess has the center offsetfrom that of the through hole, a flange part thereof may be formedaccordingly.

The drills d1 to d4 may not have same diameters as those of holes to bedrilled therewith. For example, the small-diametered drills d1,d3 may beused more than once to drill the large-diametered counterbored holes 17a,17 b.

The embodiments according to the present invention have been explainedas aforementioned. However, the embodiments of the present invention arenot limited to those explanations, and those skilled in the artascertain the essential characteristics of the present invention and canmake the various modifications and variations to the present inventionto adapt it to various usages and conditions without departing from thespirit and scope of the claims.

1. A circuit board comprising: a multilayer board, in which a pluralityof insulating layers and a plurality of conductive layers arealternately stacked; and a plurality of conductive terminals, which arefitted into a plurality of through holes extending in a thicknessdirection of the multilayer board at predetermined positions thereof,the multilayer board having cylindrical recesses each formed around athrough hole corresponding thereto, having a diameter larger than thatof the through hole, having a depth from an outermost insulating layerin the multilayer board to a surface of a predetermined conductive layerin the multilayer board, and partially exposing the surface of thepredetermined conductive layer, and the conductive layer and theconductive terminal being electrically connected at an interface betweenthe conductive layer and the conductive terminal in the through hole,and at an electrical interface between the conductive terminal and thepartially exposed surface of the conductive layer in the predeterminedconductive terminal.
 2. The circuit board according to claim 1, whereinthe conductive terminal includes a body part and a flange part.
 3. Thecircuit board according to claim 1, wherein at least either a throughhole pin or a metal plated part is provided in the through hole, and theconductive terminal is fitted into an inner surface of the through holepin or the metal plated part.
 4. The circuit board according to claim 1,wherein at least either the interface or the electrical interfaceconnects the conductive layer and the conductive terminal via conductiveconnecting material.
 5. The circuit board according to claim 4, whereinthe conductive connecting material is solder.
 6. The circuit boardaccording to claim 1, wherein a total number of the conductive layers inthe multilayer board is 2n (n is a natural number), and voltages havingopposite polarities are applied to a pair of the conductive layerspositioned above and below across the insulating layer.
 7. The circuitboard according to claim 1, wherein the cylindrical recesses are formedon both an upper surface and a lower surface of the circuit board. 8.The circuit board according to claim 2, wherein grooves for stopping ascrew is provided on an inner surface of at least either the conductiveterminal or the through hole pin.
 9. A method of manufacturing a circuitboard, in which the circuit board includes: a multilayer board, in whicha plurality of insulating layers and a plurality of conductive layersare alternately stacked; and a plurality of conductive terminals, whichare fitted into a plurality of through holes extending in a thicknessdirection of the multilayer board at a predetermined position thereof,the method comprising: a first step of forming the through holes eachpenetrating the multilayer board in a thickness direction thereof; asecond step of forming cylindrical recesses each having a diameterlarger than that of the through hole, provided by drilling from anoutermost insulating layer in the multilayer board to a surface of apredetermined conductive layer in the multilayer board, and partiallyexposing the surface of the predetermined conductive layer; a third stepof fitting the conductive terminal into the through hole correspondingthereto; and a fourth step of connecting via conductive connectingmaterial at least either between the conductive layer and the conductiveterminal fitted into the through hole, or between the conductiveterminal and the partially exposed surface of the conductive layer inthe predetermined conductive terminal.
 10. The method of manufacturing acircuit board according to claim 9, the method further comprising a stepof at least either fitting a through hole pin into the through hole ormetal plating the through hole, the step being carried out between thesecond step and the third step.
 11. The method of manufacturing acircuit board according to claim 9, wherein the fourth step ofconnecting via conductive connecting material is performed by any one ofdip soldering, reflow soldering, and flow soldering.
 12. The method ofmanufacturing a circuit board according to claim 9, wherein, in thesecond step, a pair of cylindrical recesses are formed on both an uppersurface and a lower surface of the circuit board.
 13. The method ofmanufacturing a circuit board according to claim 9, wherein theconductive terminal has a flange part on one end thereof.
 14. The methodof manufacturing a circuit board according to claim 12, wherein theconductive terminal includes a body part, a first flange part, and asecond flange part, latter two of which are provided on both sides ofthe body part; and wherein the third step is a step of setting theconductive terminal into the through hole corresponding thereto, if thecylindrical recesses are formed on both the upper surface and the lowersurface of the circuit board, in which the body part is fitted into thethrough hole; the first flange part comes in contact with the partiallyexposed surface of the conductive layer, which is exposed by one of thepair of cylindrical recesses formed on the upper surface of the circuitboard; and the second flange part, which is formed separately from thebody part, comes in contact with another partially exposed surface ofthe conductive layer, which is exposed by the other cylindrical recessformed on the lower surface of the circuit board.
 15. The method ofmanufacturing a circuit board according to claim 14, wherein the secondflange part is screwed to the body part.
 16. The method of manufacturinga circuit board according to claim 9, the method further comprising afifth step, after the fourth step, of heating the multilayer board withthe conductive terminals fitted thereinto, wherein the multilayer boardis heated with a weight put thereon.